Synthetic elastomers having a linear backbone are often employed in the manufacture of tire components, such as sidewalls and treads. It is believed that these polymers provide advantageous tensile properties, abrasion resistance, low hysteresis, and fatigue resistance. For example, cis-1,4-polydienes have been used in tires. These polymers can be produced by using lanthanide-based catalyst systems, which results in the formation of polymers characterized by a linear backbone. Polydienes having low or medium cis-1,4-linkage contents and polydiene copolymers, such as random copolymers of butadiene, styrene, and optionally isoprene, are also often employed in tires. These polymers can generally be produced by employing anionic initiators, such as n-butyllithium, which yields polymers with a linear backbone.
While synthetic elastomers having a linear backbone demonstrate a number of advantageous properties, especially for use in tires, these polymers exhibit cold flow due to their linear backbone structures. Conventionally, especially with anionically-polymerized polymers, the cold flow issues can be alleviated through polymer coupling, whereby two or more polymer chains are coupled or joined together via reaction with a coupling agent. Polymer coupling, however, presents several technological challenges. For example, the benefits associated with a reduction in cold flow must be balanced with the processability of the polymers during compounding. Also, whether certain compounds or reagents are able to react with a polymer chain, especially the reactive end of a polymer chain, can be unpredictable. Still further, it can be difficult to predict whether any particular coupling agent may have a deleterious impact upon one or more of the properties sought from the polymer and/or its use within particular compositions, such as those employed in the manufacture of tire components.
Because there is a need to reduce the cold flow of synthetic elastomers, especially linear polymers prepared by lanthanide-based catalyst systems and anionic initiators, without having a deleterious impact on the processability and/or use of the polymers, especially in the manufacture of tire components, there exists a need to develop new coupling agents and methods for coupling polymers.